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    Re: The development of bubble sextants
    From: Frank Reed
    Date: 2009 Aug 15, 08:26 -0700

    Quoting Gary:
    "Coriolis is a fictitious force use to explain the perceived curve of the 
    flight path as observed by an observer on earth of other rotating frame 
    of reference."
    
    Coriolis acceleration is a "frame-dependent" acceleration. That is, it depends 
    on the choice of coordinates that you use to define motion. The expression 
    "fictitious force" is a technical term intended to describe this 
    frame-dependence. Just so there's no misunderstanding, it DOES NOT mean that 
    the Coriolis acceleration is "fictional" in the ordinary sense of the English 
    word. Of course, Coriolis acceleration is "real" in the sense that in many 
    circumstances you find that the natural coordinates to describe the problem 
    are rotating coordinates. The weather on Earth or any other planet, for that 
    matter, is nearly impossible to understand in non-rotating coordinates but 
    quite easy to understand in coordinates which rotate.
    
    From a modern perspective (modern = since the early part of the 20th century, 
    after the development of general relativity), even the common acceleration of 
    gravity near the surface of the Earth (at a single point) at 9.8 m/s^2 or 32 
    ft/sec^2 is a "fictitious force" since it can be eliminated by going to a 
    frame of reference which is accelerating toward the Earth's center, in other 
    words a freefall frame. And sure enough, if you place an aircraft on a 
    parabolic trajectory with that acceleration, gravity disappears and 
    passengers are rendered completely weightless exactly as if they are in orbit 
    (until the plane's acceleration trajectory is changed). It is not "as if" 
    there is no gravity; in that frame of reference, there really is "no gravity" 
    apart from local tidal accelerations.
    
    Many people are able to explain the origin of the Coriolis acceleration by 
    describing how it appears in an inertial, non-rotating frame of reference. In 
    such a frame, the object moves on a straight line "while the Earth turns 
    beneath it". This is very important information, of course, but it is a 
    derivation, like an "etymology". It tells us why this acceleration must exist 
    in non-inertial frames of reference, but it doesn't mean that it is fake or 
    "fictional". And you surely wouldn't want to revert to the derivation once 
    you understand why it works. 
    
    In many ways, the expression "fictitious force" in physics has caused as many 
    problems as the expression "imaginary number" in mathematics. Neither of 
    these concepts are invalid or fictional or "un-real". For modern mathematics, 
    so-called imaginary numbers have no defects, no lack of reality to them. They 
    are what they are: solutions to algebraic equations. Likewise in physics, 
    so-called fictitious forces are not forces to be avoided or treated as 
    "un-real". They are what they are: forces arising from the choice of 
    coordinates.
    
    -FER
    
    
    
    
    
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